Improve solution provider repo interface

[AlexVanderbist]

This PR updates the (internal) SolutionProviderRepository interface to allow instances of SolutionProviders to be passed to the solution provider repo like the doc blocks suggest.

Closes #121

[AlexVanderbist]

Failing tests are not relevant to this PR.

[Jubeki]

@AlexVanderbist

Just to make you aware, this broke spatie/laravel-ignition because Spatie\LaravelIgnition\Solutions\SolutionProviders\SolutionProviderRepository also implements the Contract SolutionProviderRepositoryContract

[mohamedyasser]

@AlexVanderbist

Just a confirmation for @Jubeki comment it broke
Spatie\LaravelIgnition\Solutions\SolutionProviders\SolutionProviderRepository::registerSolutionProvider
it must be compatible with Contract SolutionProviderRepository

Whoops\Exception\ErrorException

Declaration of Spatie\LaravelIgnition\Solutions\SolutionProviders\SolutionProviderRepository::registerSolutionProvider(string $solutionProviderClass): Spatie\Ignition\Contracts\SolutionProviderRepository must be compatible with Spatie\Ignition\Contracts\SolutionProviderRepository::registerSolutionProvider(Spatie\Ignition\Contracts\HasSolutionsForThrowable|string $solutionProvider): Spatie\Ignition\Contracts\SolutionProviderRepository

at vendor/spatie/laravel-ignition/src/Solutions/SolutionProviders/SolutionProviderRepository.php:24
20▕ {
21▕ $this->solutionProviders = Collection::make($solutionProviders);
22▕ }
23▕
➜ 24▕ public function registerSolutionProvider(string $solutionProviderClass): SolutionProviderRepositoryContract
25▕ {
26▕ $this->solutionProviders->push($solutionProviderClass);
27▕
28▕ return $this;